Process of treating iron-silicon alloys



Patented Apr. 5, 19 32 MICHAEL GEORGE CORSON, OF NEW YORK, N. Y.,

EN'T OFFICE INCL, 0F DAYTON, OHIO, A CORPORATION NEW YORK PROCESS OI TREA'IQIING IRON-SILIOON ALLOYS A No Drawing. Application filed May 9, 1928, Serial No. 276,502. Renewed February 20,

The invention relates to iron-silicon alloys and to a process of treating same. In particular, the invention relates to the treatment of iron-silicon alloys containing upwards of 5% silicon to render them workable and impart ductility thereto.

It has been known for a long time that iron-silicon alloys become practically unforgeable when the silicon content is ralsed above about 5%. However, the alloys containing higher percentages of silicon possess certain properties, namely high electricresistivit and resistance to corrosion, which, if com ined with ductility, would render them desirable for various uses.

' I have found that the brittleness of the iron-silicon alloys is due to two causes. The first is that silicon forms with iron a. compound Fe Si which is characterized by hardness and lack of ductility. Therefore, when the silicon is added to pure iron in increasing proportion up to 14.4% silicon the alloys change from a very ductile metal to an unwieldy compound. This entails a continuous decrease in ductility and increase in hardness.

Secondly, all these alloys have to pass through a double transformation when cooling from the casting temperature. 7 At temperatures included between 1400 C. and

10%0 C., the actual temperature at which the transformation begins depending on the composition and varying as to the upper limit inversely with the proportion of silicdn, these alloys begin to change from a uni form solid solution, the delta phase, to a dulexstructure of delta plus gamma the main ody of each grain becoming enriched with silicon, a 3

At temperatures ranging from 900 to l040 C. the alloys go through an opposite transformation, which, if the temperature is maintained within the desired range a sufii cient length of time, changes the metal back into a uniform solid solution. If such time is not allowed, the retransformation .to the condition of a uniform solid solution remains incomplete'and this incompleteness, in addi- "tion to the weakness or brittleness due to the presence of the duplex structure, causes additional weakness of-the grains along their slip planes. It is for these reasons that the tendency of these alloys to break under every effort 0 forgin or rolling is to be attributed. The ingots cooling down in the molds do not have time enough to pass evenly and comletely through the two transformations.

ooling of the incompletelytransformed in- .ASSIGNOR TO THE DURIRON COMPANY,

gots causes them to develop ultramicroscopic cracks and these'do not weld together when the ingots are reheated for forging.

Accordingly, duct the casting, cooling and working of the metal as to insure completion of the double transformation and to prevent the formation my process aims to so conof ultramlcroscopic cracks in'the metal and the development of a structure causing brit tleness and lack of strength.

I have also found that nickel, when present in small amounts, not exceeding 1%," and usually less than .5%, acts as a catalyzer during the two transformations described, thereby accelerating the rate of transformation and substantially shortening the time during which the temperature of the metal is to be closely controlled,-as well as further improving the ductility and workability of the metal. 0 The invention, therefore, includes as a modification the addition of small amounts of nickel for the purpose above mentioned.

The invention comprises broadly the casting, cooling and working, either by forging orrolling, of iron-silicon alloyscontaining. upwards of 5% and preferably not exceeding about 14.4%silicon, under such conditionsas will insure the absence of transformations and prevent the appearance of a duplex structure during the process of working. The invention further includes the cooling of alloys of the class above-described, either after working above the two transformation zones or, directly from the casting operation with-.

out working at such a rate and under such conditions asto insure the retransformation to a uniform solid solution or delta phase of 4 the delta plus gamma structure which may have formed when passing through the first transformation zone, thereby preventing the development of a structure causing brittleness and lack of strength.

The metal may be cooled from the casting temperature su ciently slowly when passing 5 through the tem erature ranges at which the respective trans ormations take place to insure completion of each transformation before cooling to a lower temperature, thereby producing a uniform solid solution. Or, since. it is not essential that the first transformation,-from delta to delta plus gamma,-'shall have been permitted to become complete before the retransformation to delta phase or a uniform solid solution begins, the metal in cooling may be passed either slowly or quickly through the range at which the first transformation takes place so long as it is held for a sufliciently long time in the range where the second transformation takes go place to insure its completion. The metal may then be reheated and worked to the desired extent. Since cooling of the metal from the casting temperature to the cold condition under the above conditions requirea.

5 a relatively long controlled heat treatments usually at least 24 hours, it is found more feasible from a practical standpoint, to carry out the working of the metal before the ingots have cooled to below the working tempera-' so ture. The necessity forreheating is also thereby eliminated.

According to a preferred procedure for carrying out the invention, an iron-silicon alloy of any desired composition within the range $5 specified is prepared, preferably under conditions insuring the absence of carbon in the product. The molten alloy is poured into molds preheated preferably to at least 850. C. or to such a temperature as will insure that 40 the outer portions of the ingot will not have become chilled to a point where the first for mation will begin before the ingot has become solid enough to be removed fromthe mold. As soon as the ingot has solidified sufficiently to be removed from the mold, it is placed in a furnace and subjected to a soaking treatment while maintaining the temperature of the furnace above the temperature at which the first transformation'begins until the temperature throughout the ingot has become equalized. For example, when treatin I a. 12% silicon alloy the furnace temperature should-be above about 1050 C. When treating alloys containing between 6 and 11% fissilicon the furnace temperature preferably "should be about 100 higher or above approximately 1150 C. i The metal is permitted to remain in the furnace until it is desired to start the work- 'ing operation, when the ingot is immediatelyrolled or forgedwhile hot. The temperature of the metal during working preferably should not be permitted to fall to the point where the first transformation begins,

I although a slight under cooling may take place without detriment providing the transformation is quickly checked by reheating. Therefore, the temperature of the metal at the beginning of the working operation should be considerably above the minimum soaking temperatures above-mentioned in order to allow for cooling during working and if considerable work is to be performed upon the metal it may require reheating treatments between successive working steps to insure the maintenance of an optimum working temperature of the metal as aboveindicated. After its shape has been changed by the application of mechanical work to the desired degree, the worked turned to the furnace which is closed completely and the temperature lowered by small steps until a temperature of 500 C is reached. This procedure renders the alloy reasonably ductile at lowtemperature so that the drop in ductility is much less pronounced than that observed in iron-silicon alloys produced in the normal way as the silicon content increases from the substantially pure iron to the Fe si' compound.

The cooling of the metal at a controlled rate after the working operation is important, particularly through the range at which the second transformation will take place;

since this second transformation proceeds slowly. However, as it continues far below its point of beginning its completion may be insured and the cooling of the metal expedited by cooling the wrought metal through such range gradually and at such a rate that such transformation will have become completed when the temperature has dropped to the point where the transformation will no longer take place. I have ascertained this point to be about 500 C. with iron-silicon alloys" of theclass above-mentioned. Good results are insured by cooling these alloys from 900 to 500 C. in approximately 12 hours.

As indicated above,I have found that nickel in small amounts from about 0.2% to 1%, and usually less than .5%,' acts as a catalyzer during the two transformations described, thereby shortening the heat treatment con- -'siderably and effecting improvement in the properties of the alloy. Accordingly, I prefer to introduce small amounts of nickel into the alloy, although it is not essential that nickel be added'in order to produce a marked improvement in the ductility and workability of the alloy. v x i Y I have indicated abdve that it is prefer able to produce the alloy under conditions that insure the absence of carbon in the resulting product, owing to the bad influence of carbon on the workability of alloys of the type here considered. However, in certain cases it may be desirable to use a carbon-containing scrap in making the alloy. I have found that the best way to eliminate the bad material is re influence of carbon is to introduce a metal with which it will combine in the presence of silicon to form coalesced carbides instead of films and flakes of graphite. Most oarbideforming elements tend in the presence of silicon to stay in solid solution in the iron. I have found, however, that vanadium forms carbides in the presence of silicon and that when it is present in small amounts up to 1% the properties of the carbon-containing alloys are substantially improved due to conversion of the carbon into the carbide form. When approximately 1% of vanadium is present it will eliminate as much as .15% carbon by converting it into carbiden T It will be understood that various changes may be made in the details of the procedure outlined and in the composition of the alloy without departing from the invention which is not to be deemed as limited other thanas indicated in the appended claims,

, What I claim is:

1. Process of making wrought articles of an iron-silicon alloy containing. upwards of 5% silicon which comprises casting said alloy into a mold preheated to at least 850 C., removing the resultin ingot from said mold when it has cooled su ciently to be selfsustaining and before transformation of the metal from a state of; solid solution has taken place, soakingsaid ingot at a temperature of at least 1050 (1., working said ingot to produce articles of the desired form and size while maintaining the temperature thereof above the point where ultramicroscopic cracks begin to develop on rapid cooling, and finally gradually lowering the temperature to a point where rapid cooling does not afi'ect the properties of the metal,

a 2. Process of makingwrought articles of an iron-silicon alloy containing hpwards of 5% silicon which comprises casting and'then working said alloy to the desired form and size without permitting the same to cool below about 1040 C. at any time between casting and completion of the working step, and

then cooling gradually to a point where rapid cooling does not materially affect the properties of the metal.

3. Process of making wrought articles of an iron-silicon alloy containing upwards of 5% silicon which comprises casting and then V soaking and workingsaid alloy to the desired form and size without permitting the same to cool below about 1040 C. at any time between casting and completion of the working step,

and finally cooling gradually to a point where rapid cooling does not materially affect the properties of the metal.

4. Process of producing a workable.ironsilicon alloy containing upwards of 5% silicon which comprises casting the alloy into a mold preheated to at least 850 (3., then immediately after removing the resulting metal,

from the mold bringing it to a substantially uniform temperature throughout of at least 1050 0., and then cooling to a point below 900 C. at a sufliciently low rate as to substantially prevent the formation of ultramicro: scopic cracks,

5. Process of producing a workable ironsilicon alloy containing upwards of 5% silicon which comprises casting the alloy in a preheated mold, removing the'resulting ingot from the mold before it has cooled below the point at which the uniform solid solution,- the delta phase-starts to change to a duplex structureof delta plus gamma, heating the metal while still heated as aforesaid at above 1050 C. and then cooling at a sufiiciently slow rate as to substantially prevent the formation of ultramicroscopic cracks.

6. Process of producing wrought articles of iron-silicon alloys containing from 6 to 11% silicon which comprises casting the alloy, removing the resultin ingot from the mold before it has cooled helow the point where the uniform solid solution begins to change to a duplex structure of delta plus gamma, then without further cooling heating the metal at above 1150 C. to effect strucupwards of 5% silicon which comprises adding nickel in small amounts up to 1%, casting the alloy in a mold preheated to at least 850 (1, removin the resulting ingot from the said mold wien it has cooled sufliciently to be self-sustaining, then heating said ingot at a temperature of at least 1050 C. until ,the structural homogenization which takes place above said temperature has become complete and then cooling the metal sufficiently slowly to below 9005C. as to insure that the metal is brought into a state of substantially uniform solid solution. t "F i 8. Process of making wrought articles of iron-silicon alloys containing upwards of 5% silicon which comprises forming an alloy containing iron and silicon in the desired proportions, adding vanadium in sufficient amountsto convert any carbon present to the form of a carbide, casting the alloy, removing the resulting ingot from the mold when I throughout thecasting, cooling, heating and T working steps being maintained above about 1040 C.

9. Process of improving the ductility and workability of iron-silicon alloys containing upwards of 5% silicon which comprises forming an alloy containing iron and silicon in the desired proportions, adding vanadium in small amounts up to 1% to convert any car bon present to the form of carbides, casting the alloy, removing the resulting ingot from the mold when it has cooled sufficiently to become self-sustaining, then heating the ingot at a temperature above the point where transformation from the delta phase to delta plus gamma begins, to effect structural homogenization of the metal, the rate of cooling throu h; the range at which retransformation of the delta plus gamma structure to a uniform solid solution will take lace being so controlled as to insure comp etion of such retransformation.

10. Process of improving the ductility and I workability of iron-silicon alloys containing upwards of 5% silicon which comprises forming an alloy containing iron and silicon in the desired proportions, adding vanadium in sufiicient amount to conyert any carbon present to the carbide form, casting the alloy in 'a mold preheated to at least 850 (1, removin the resulting ingot from the mold when it as cooled sufiiciently to become self-sustaining, then heating the ingot to at least 1050 C. until the structural homogenization which takes place above said temperature has become complete, and then cooling the metal sufficiently slowly through the range 900 C.500 C. to insure conversion of any delta plus gamma structure formed during previous stages into a state of substantially uniform solid solution.

11. Process of improving the ductility and workability of iron-silicon alloys containing upwards of 5% silicon which comprises casting the alloy into a heated mold, removing the resulting ingot from the mold before the temperature thereof has dropped to 900 0., heating the ingot to at least 1050 C. and then hot working.

12. A wrought metal article consistin principally of iron and containing from 5% to about 14% silicon, andcharacterized by aphysical structure and properties such as are produced when an alloy of like composition is cast in a mold preheated to at least 850 C., removed while hot, and then soaked and worked to the desired form and size without permitting the same to cool below about 1040 (3., followed by gradual cooling to a point below 900 C. c

13. A metal article consisting principally of iron and containing from 5% to about 14% silicon, and characterized by a physical structure and properties such as are produced when an alloy of like composition is cast in a preheated mold, removed from the and then cooling,

lot

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